1. Field of the Invention
The present invention relates to an optical recording medium having a write once type recording layer and a method of manufacturing the same.
2. Discussion of Related Art
Conventionally known is an optical recording medium provided with an underlayer formed under a write once type recording layer, the underlayer being composed of a compound which causes thermal deformation such as explosion, dissolution, evaporation, decomposition, sublimation or the like at a temperature lower than the melting temperature of the recording layer.
In this specification, hereinafter, a compound which causes the above-described thermal deformation, when it is formed as an underlayer of a recording medium, is referred to as a thermally deforming compound.
The method of manufacturing such an underlayer includes a method of directly depositing the above thermally deforming compound on the recording signal pattern surface of a substrate by any means and an extension and exfoliation method. As the depositing method a spin coating method and the like are used.
When the spin depositing method is carried out, a solution of a thermally deforming compound dissolved in a suitable solvent is directly spin coated on the recording signal pattern surface of a transparent substrate.
Depositing a film by a method not limited to the spin coating method is disclosed, for example, in Japanese Patent Kokoku (Post Exam. Publication) No. Sho 62-27457 (U.S. Pat. No. 4,405,862). According to this method, holes are more effectively defined by combining a heat sensitive film (corresponding to the film of the thermally deforming compound) with a film (metal or semi-metal layer) having a smaller thickness and, absorbing energy property. In the disclosure, each of the heat sensitive film and the energy absorbing film of the combined films is on the side of the substrate.
The extension method is a new technology which was recently developed and disclosed in United States Patent Application filed on Jul. 20, 1990. The extension method is such that a solution of a first organic compound, i.e., a thermally deforming compound dissolved in a suitable solvent, is spin coated on the recording signal pattern surface of a recording medium making metal mold referred to as a stamper, and dried, semi-dried or remains undried to form an underlayer. Then a liquid second organic compound which is a curable resin material is placed on a portion of the underlayer, and a transparent substrate is pressed against the liquid resin material to expand it to a uniform thickness. Thereafter, the resin material is cured and the coated layer is exfoliated from the above stamper at the interface thereof so that the underlayer is integrally formed on the surface of the resin layer.
According to the extension method, when the recording medium is produced, the mixed member of the resin material and the above thermally deforming compound forming the underlayer makes contact with the recording signal pattern surface of the stamper. Therefore, it was discovered that a closely adhering force of the stamper with a replica was lowered as compared with the case in which a single layer of a thermally deforming compound makes contact with the signal surface of the stamper.
Consequently, the extension method and the formation of the mixed layer of the underlayer composed of the thermally deforming compound and the resin material are effective in that the replica is easily exfoliated from the stamper and deformation of a preformat pattern is prevented. Further, since an excessive exfoliating force is not necessary to be applied to the transparent substrate, the substrate and replica layer can be prevented from being broken, and thus this technology has an excellent meritorious effect.
The underlayer formed by the spin coating method from the above two underlayer forming methods is weak in respect to a closely adhering force with the transparent substrate and is liable to be easily exfoliated under the circumstances of high temperature and high humidity.
With respect to the underlayer formed by the depositing method of the above two underlayer forming methods, first, the underlayer formed by the spin coating method is liable to be exfoliated, in particular, under the circumstances of high temperature and high humidity, and the like, because a means for increasing the intimate adhesion strength with the transparent substrate is not present.
Further, a heat sensitive (thermally deforming) film is formed at the location next to a transparent substrate or an energy absorbing film (metal or semi-metal) in the case of an underlayer formed by the not specified depositing method disclosed, for example, in U.S. Pat. No. 4,405,862, but in this case a means for increasing the intimate adhesion strength between the underlayer and the transparent substrate or between the underlayer and energy absorbing film is not disclosed. Therefore, the heat sensitive (thermally deforming) film and the transparent substrate are also easily exfoliated from the underlayer under the circumstances of high temperature and high humidity.
On the other hand, in the case of the underlayer formed by the extension method, since the resin material forming the resin layer permeates into the compound forming the underlayer, the underlayer has a high bonding strength with the resin layer, and moreover the surface of the transparent substrate is subject to a silane treatment or the like so that the resin layer is strongly adhered to the transparent substrate, and thus finally the underlayer is strongly adhered to the transparent substrate.
Nevertheless, according to this method, a problem arises in that since the resin material forming the resin layer permeates into the underlayer and a part thereof is exposed on the surface of the underlayer (surface on which a recording layer is formed), the concentration of the compound in the underlayer is lowered, whereby curing of the underlayer is weakened which lowers recording sensitivity.
More specifically, although a metal or resin stamper has been conventionally known as the above stamper, the metal stamper (for example, a nickel stamper) has been used, because it is excellent in durability.
Nevertheless, there is a very strong intimate adhesion strength between the above metal stamper and a compound forming the above underlayer, and thus when an underlayer as a single layer composed of the above compound only is formed, a large exfoliating force must be applied to exfoliate a replica from the stamper. As a result, there is a drawback in that a transferred preformat pattern is deformed or a transparent substrate is damaged in the worse case.
Thus, when the replica is made, a resin material having a weak adhesion strength must be exposed on the surface in contact with the stamper in such a manner that a mixed member of the above resin material and compound is extended on the stamper, or a solution of the above compound is spin coated on the stamper and then a resin material having strong permeation into the compound is extended on the coated layer, which causes such problems as the lowering of sensitivity as described above, and the like.